Yet it allows to build native applications where UI structure is defined by HTML, style by CSS and reactivity by script or by Rust (or by C++, Go, Python, etc.)
1. FlexBox breaks existing CSS Box model that mandates that dimensions of the element are defined by its width/height properties.
2. FlexBox introduces 12 (sic!) new properties in already overcrowded CSS property map. But as demonstrated by Sciter that can be achieved by just one property `flow` and flex units.
3. Flexbox and Grid are conflicting in the sense that the flexibility (as a feature) is defined in two different ways: separate properties (FlexBox) and 'fr' units (Grid) - overall CSS architecture becomes a zoo. Yet CSS has that famous 'auto' value that in some cases is just 1fr ( 1* in Sciter terms).
4. FlexBox is applied through 'display' property that is conceptually wrong. 'display' specifies how the element itself is replaced among its siblings but flexbox is rather about layout of element's children. These two entities are orthogonal. Example:
display:list-item; or display:table-cell; cannot be flexboxes. Just because of architectural specification error but physically nothing prevents <td> to use flexbox layout for their children.
5. Flexbox arrived too late. We started talking about the feature 12 years ago on www-style WG at W3C. And all these years Sciter already used flexibility (the must for desktop UI).
Not the parent, but in my view it is basically a 1-dimensional subset of Grid, which is great for mostly-1D website design.
But when it comes to making applications (which often use the whole screen, with tabs and whatnot, instead of being scroll-oriented), it makes some things either really difficult, either still requiring custom code to get it just right, or having a handful of wrapper elements for relatively simple layouts.
Don't get me wrong, it's _miles_ better for web layout than the old systems, but since Sciter seems to be focused on application-style layouts, it makes sense to prefer a layout system that can better represent common patterns in that domain.
Also, if you plan on go responsive, the required flex wrappers kind of hold you back when you need to reorient a bunch of your layout to fit a vertical screen (or horizontal, if you're doing mobile-first), while Grid's `grid-area` is beautifully flexible in that regard.
TL;DR: If you're gonna have to have Grid, then just having Grid is perfectly fine as it can do everything Flex can and more.
However it has one major issue : it's proprietary... And that's a no go for many project / companies.
I would not make my entire business depend on a blob which I have no control on it.